This invention relates to a paint composition of an aqueous slurry type the solid component of which is a powder paint.
Much attention and studies have already been given to aqueous paints of the type comprising a powder paint dispersed in water. A vital technological problem about this type of paints is how to realize satisfactorily stable dispersion of solid paint particles in water. One of conventionally prevailing measures for this purpose is to extremely reduce the size of paint particles, for example to the extent of about 5 microns, in advance of dispersing the particles in water. However, it takes much time and is not easy to accomplish atomization of a powder paint into desirably fine particles, and, furthermore, even though extremely fine paint particles are employed gradual sedimentation of the paint particles which are inherently insoluble in water is inevitable when the paint is left standing for a long period of time, resulting in that an initially realized uniform dispersion phase separates into a very dilute upper layer and an extremely solid-rich lower layer, meaning that the paint is no longer of practical use.
Another measure for stabilizing dispersion of paint particles in water is to add a certain dispersion-promoting agent such as a surfactant to the paint. This measure is easy to perform and quite effective for realizing a uniform dispersion phase and preventing sedimentation of paint particles. However, the presence of a surfactant in the aqueous paint causes paint films formed by the application of this paint to become considerably inferior in durability and poor in water resistance. Therefore, satisfaction has not yet been reached by the use of a surfactant either.
In various fields of industries where powder paints are used, another matter of concern is that the reuse of recovered powder paints is difficult. In general, the merits of using powder paints reside in little fear of creating pollution problems and much possibility of recovering and reusing the paints. In practice, however, the reuse of recovered powder paints become often impracticable. For example, where articles are painted in a variety of colors as in the final coat of automobiles or household electric appliances, powder paints are recovered in a state of a compound color paint and, hence, can hardly be reused in the recovered state. Therefore, powder paints recovered in such fields of industries are now used merely as fillers or extenders for plastic moulding materials, dedonors, rubbers, etc. Of course, such a manner of reuse cannot be deemed worthy of powder paints which are relatively high-priced materials.